1. Field of the Invention
The present invention relates to a liquid ejection head, a liquid ejection apparatus and an image forming apparatus, and more particularly, to technology for preventing ejection defects caused by increased viscosity of the ink inside nozzles.
2. Description of the Related Art
An inkjet recording apparatus records a desired image on a recording medium by ejecting ink droplets selectively from nozzles in a recording head, while moving the recording head having the nozzles relatively with respect to the recording medium. Recording apparatuses of this kind are broadly divided into line type apparatuses and serial type apparatuses. The line type apparatuses carry out recording simply by moving a recording medium and a long recording head (line head) having substantially the same width as the recording medium, relatively to each other, in the paper conveyance direction (sub-scanning direction), and the serial type apparatuses carry out recording by moving a short recording head (shuttle head) back and forth reciprocally in the breadthways direction of the recording medium (main scanning direction). Furthermore, the ink ejection method may be, for example, a piezoelectric method, which ejects ink droplets from a nozzle by using the displacement of a piezoelectric element to pressurize the ink in a pressure chamber, or a thermal method, which generates bubbles inside a pressure chamber by means of the thermal energy produced by a heating element, such as a heater, an ink droplet being ejected from a nozzle due to the pressure generated by these bubbles.
In an inkjet recording apparatus of this kind, if variations arise in the volume, flight speed, flight direction, or the like, of the ink droplets ejected from the nozzles and ejection becomes instable, then this may lead to deterioration of the image quality. Hence, in order to improve the image quality, it is extremely important that ink droplets can be ejected in a stable state at all times from the nozzles.
Possible examples of factors which cause ejection instabilities are contact between the recording medium and the nozzle surface of the recording head, and adherence of foreign material such as dust, dirt, ink droplets, or the like, to the nozzle surface. Therefore, in order to protect the nozzle surface, for example, in Japanese Patent Application Publication No. 2001-018390, a nozzle protection plate formed with slit-shaped openings that are to be ink ejection channels is provided at a prescribed interval from the nozzle surface.
Another factor which causes ejection instabilities is drying or increase in the viscosity of the ink inside the nozzles when ink is not ejected. Therefore, in order to prevent ejection defects due to increased viscosity of the ink, for example, in Japanese Patent Application Publication No. 06-226985, a cap member is placed in close contact with the nozzle surface, and in Japanese Patent Application Publication No. 2000-127387, the nozzles are closed off with a sealing liquid.
However, in Japanese Patent Application Publication No. 2001-018390, in addition to protecting the nozzle surface from contact or impacts with the recording medium, or adherence of foreign material, or the like, it is also sought to prevent ink droplets from collecting on the nozzle protection plate or the nozzle surface by designing the shape of the openings in order that the ink droplets can pass through same, but no consideration is given to the evaporation of the ink solvent from the interior of the nozzles though these openings. Consequently, there is a problem in that although the nozzle surface can be protected, it is not possible to prevent ejection defects due to increased viscosity of the ink.
In Japanese Patent Application Publication No. 06-226985, a serial type of inkjet recording apparatus is used and when the apparatus has changed from a recording state to a non-recording state, the recording head is moved to a non-recording region, and the cap member disposed in that region is moved and placed in close contact with the nozzle surface of the recording head. However, if it is sought to apply a composition of this kind to a line type of inkjet recording apparatus, this leads to increased size of the apparatus and increased costs, in addition to which a long time is required until the cap member is placed in close contact with the nozzle surface, and evaporation of the ink solvent progresses during this time. Moreover, even after the cap member has been placed in close contact with the nozzle surface, there is also a risk that evaporation of the ink solvent will continue until the sealed air enclosed by the cap member reaches a state saturated with the evaporated ink solvent. In this way, it takes a long time to restrict the progress of the evaporation of the ink solvent inside the nozzles, and therefore ejection defects may arise as a result of increased ink viscosity. Consequently, when recording is restarted, it becomes necessary to remove the ink of increased viscosity inside the nozzles by suctioning or preliminary ejection (also called purging or spit ejection), and hence a large amount of ink is consumed wastefully.
In Japanese Patent Application Publication No. 2000-127387, a composition for supplying the sealing liquid is required, and this leads to increased size of the apparatus and increased costs. Moreover, since the sealing liquid is a liquid that is different to the ink components, then when the sealing liquid is removed before restarting recording, the ink must also be removed at the same time, leading to a problem in that a large amount of ink is consumed wastefully